This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to a low cost flat faceplate and front assembly, and method of production therefor, for use in a color CRT having a flat tensioned shadow mask.
Most glass produced in the past has been in the form of sheet, window or drawn glass which is characterized as having two smooth or fire polished surfaces which typically require no further processing once produced. Over the years, this has been the most important type of flat glass for the construction industry and is used primarily for windows. This type of glass is initially formed as plate glass by drawing the glass vertically from a tank by means of a metal "bait", whereupon the glass is immediately contacted by cooled side-rollers to prevent it from flowing together and is then directed horizontally before entering an annealing lehr. The degree of flatness and parallelism achieved by thus rolling the malleable glass is of a relatively high standard, which is typically improved by optical methods such as grinding, polishing and acid etching.
Since the 1960s, however, sheet glass has increasingly been replaced by float glass which is manufactured by a process wherein the glass floats like an endless ribbon on a bath of molten tin to produce high quality glass having flat surface planes. Float glass has been used for glazing whenever transparency is required in buildings and is also used as a raw material for making safety glass, mirrors and otherwise finished or processed flat glass for furniture as well as for liquid crystal diode (LCD) displays.
Video displays such as CRTs have a glass construction which to date has not been amenable to either of the aforementioned glass production techniques. A CRT generally consists of an evacuated envelope having a neck portion, a faceplate, and a funnel portion therebetween. An electron gun disposed in the neck portion of the envelope emits energetic electrons which are directed onto the inner surface of the faceplate. Disposed on the inner surface of the faceplate are a large number of phosphor elements which glow momentarily when struck from the rear by electrons from the electron gun to produce a video image which is visible through the faceplate.
Prior art CRTs have generally been of the curved faceplate type wherein the faceplate is generally convex as seen by a viewer. The faceplate is initially formed in a pressing mold which typically includes a hollow mold into which a glass gob is deposited. The glass gob is then hydraulically or pneumatically pressed by a sealing ring-guided plunger until the glass is pressed into all areas of the mold and assumes the desired curvature. After solidification, the plunger is withdrawn and the solid glass faceplate is remcved from the hollow mold. One example of a method of press shaping glass faceplates using a rotary press-molding machine is disclosed in U.S. Pat. No. 3,615,328 to Coleman. This method as well as other prior art approaches involves the step-wise, sequential fabrication of individual faceplates and requires long periods of pressing time and is thus lengthy and expensive.
Recent developments in video displays have led to a color CRT having a substantially flat faceplate and incorporating a shadow mask of the tensioned foil type which offers various advantages over the aforementioned prior art curved faceplate CRTs including improved brightness and/or contrast of the video image. Although not subjected to the curvature inducing sagging operation, flat faceplates for use in CRTs having flat tensioned shadow masks are also generally formed by the aforementioned pressing process. As a result, flat faceplates as in the case of the aforementioned curved CRT faceplates have not yet been amenable to low cost, simplified production techniques.
The commercial success of CRTs employing a substantially flat faceplate depends, in large part, upon the cost to manufacture this type of CRT. Much effort has thus been expended to reduce the cost of CRTs having a substantially flat faceplate and incorporating a shadow mask of the flat tensioned foil type. The present invention is the result of such efforts in that it contemplates a method of producing, as well as the composition of, a flat glass faceplate for a color CRT which substantially simplifies and reduces the cost of the faceplate, while providing a high degree of video image acuity.